doc/go1.13: describe the change in behavior of GO111MODULE=auto

Updates #31857

Change-Id: Id0dc66246f768a2c730c67cd7a4a7830547f22a8
Reviewed-on: https://go-review.googlesource.com/c/go/+/183922
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Katie Hockman <katie@golang.org>

doc/go1.13.html

@@ -169,6 +169,17 @@ <h3 id="modules">Modules</h3>
 
 <h4 id="proxy-vars">Environment variables</h4>
 
+<p><!-- CL 176580 -->
+  The <a href="/cmd/go/#hdr-Module_support"><code>GO111MODULE</code></a>
+  environment variable continues to default to <code>auto</code>, but
+  the <code>auto</code> setting now activates the module-aware mode of
+  the <code>go</code> command whenever the current working directory contains,
+  or is below a directory containing, a <code>go.mod</code> file — even if the
+  current directory is within <code>GOPATH/src</code>. This change simplifies
+  the migration of existing code within <code>GOPATH/src</code> and the ongoing
+  maintenance of module-aware packages alongside non-module-aware importers.
+</p>
+
 <p><!-- CL 181719 -->
   The new
   <a href="/cmd/go/#hdr-Module_configuration_for_non_public_modules"><code>GOPRIVATE</code></a>